Considering buying a new phone.

Hello, I'm new to cell phone technology so I was just going to ask a few questions before I went off and purchased a new phone off of ebay or something.

I am a Cingular customer and I have a Sony Ericsson Z500a camera phone that I am marginally happy with, but I just kind of want a change. I get a chance to upgrade a year from now (which by the way I have never gone through, so I don't even know how that works) but I don't really want to wait that long. I mainly want one that is thinner than my current one because I have my phone in my pocket virtually all the time.

I have been looking at a few phones like the Motorola SLVR, but I think I should figure out what I need to look for before I just go for style. Do I just need a phone that is "Unlocked" so my SIM card will work in it, and that's it? Also, I was trying to figure out what Tri-Band / Quad-band / GSM 850 / 900 / 1800 / 1900 all means.

Also, can I get an opinion on the SLVR L6 vs. the L7?

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When you upgrade to get a new phone, you sign either a one year or 2 year contract. Sometimes they charge you extra for the phone (20-50$) more than what it would be on a new contract. The L6 is basically the SLVR L7 it jus doesnt have iTunes or a TFlash card slot on it.